Update TiDB Cloud dashboards for Dedicated, Premium, and Essential

[Yujie-Xie]

What changed

• Update the TiDB Cloud Dedicated dashboard CPU panels to use the precomputed tidb_cloud.node_cpu_seconds_total_rate metric for TiDB, TiKV, and TiFlash.
• Add dedicated Datadog dashboard assets for TiDB Cloud Premium and Essential.
• Register the Premium and Essential dashboards in the TiDB Cloud integration manifest.
• Add the changes to the TiDB Cloud 1.3.0 changelog.
• Essential V2 is intentionally not included in this change.

Why

The Dedicated dashboard still calculated CPU rate in Datadog from tidb_cloud.node_cpu_seconds_total with .as_rate().rollup(sum, 120), while the metrics integration now exports the CPU rate directly. Premium and Essential also need plan-specific dashboards matching their supported metric sets.

Impact

Users can import plan-specific Premium and Essential dashboards, and the Dedicated CPU panels use the exported CPU-rate metric consistently.

Validation

• Parsed the integration manifest and all dashboard assets with jq.
• Verified every dashboard path registered in the manifest exists.
• Verified the Dedicated dashboard changes only the three CPU queries.
• Verified the Premium dashboard contains 21 metric queries and the Essential dashboard contains 19 metric queries.
• Ran git diff --check.

[Yujie-Xie]

@bgoldberg122 Could you please review this PR? Thanks!